Chemical vapor deposition (CVD) is defined as the formation of a non-volatile solid film on a substrate by the reaction of vapor phase reactants that contain desired components. The gases are introduced into a reactor vessel, and decompose and reacted at a heated surface on the wafer to form the desired film. CVD is but one process of providing thin films on semiconductor wafers, such as films of elemental metals or compounds. It is a favored deposition process in many respects, principally because of its ability to provide highly conformal layers even within deep contacts and other openings.
CVD is generally classified into one of three types. The first two are principally predicated upon reactor pressure, and are designated as atmospheric pressure chemical vapor deposition (APCVD) or low pressure chemical vapor deposition (LPCVD).
A third category is referred to as plasma enhanced chemical vapor deposition (PECVD). Rather than relying solely on thermal energy to initiate and sustain chemical reactions, PECVD uses a radio frequency (RF) induced glow discharge to transfer energy into the reactant gases, allowing the substrate to remain at lower temperature than in APCVD or LPCVD processes. Specifically, the plasma-inducing glow discharge is generated by the application of an RF field to a low pressure gas, thereby creating free- electrons within the discharge region. The electrons gain sufficient energy from the electric field so that when they collide with gas molecules, gas-phase dissociation and ionization of the reactant gases (i.e., inducement into the plasma state) then occurs. Lower substrate temperature is the major advantage of PECVD, and provides a method of depositing films on some substrates which do not have the thermal stability to accept coating by other methods. In addition, PECVD can enhance the deposition rate when compared to thermal reactions alone, and produces films of unique compositions and properties.
One type of PECVD reactor employs a quartz reactor shell having electrically conductive inductive coils circling externally thereabout. A second electrode is received internally within the quartz shell, enabling inductive coupling to generate plasma inside of the reactor shell. Inductive coupling of power to generate the plasma refers to the transmission of power via electromagnetic radiation into the closed reactor volume having gas present therein at low pressure. The characteristics of such power transfer are very low mean ion energy, very high plasma density, and very little ion acceleration to the quartz walls. Inductive sources are very efficient in the low pressure regime, and are favored for small geometry definition.
In most inductively coupled PECVD processes, it is desirable to shield the internal reactor volume from capacitive electrostatic charge. Such is typically accomplished by the placement of an metal electrostatic shield intermediate the reactor cavity and inductive coils. Such a shield typically constitutes a metal plate having holes or elongated slots provided therethrough which shields the reactor cavity from capacitive electrostatic charge during semiconductor wafer processing. To function in this manner, such shield must be provided at ground potential, and accordingly these shields are permanently connected to ground.
An alternate technique for generating plasma is by capacitive coupling. Here, capacitive electrostatic charge is purposefully utilized to create strong electric fields at the electrode locations, which in turn creates a plasma sheath region. This type of power generation has the characteristic of heavy ion acceleration toward the negative electrode and very low plasma density. Plasma generation in this manner is typically utilized in dry chemical etching. As opposed to deposition as in CVD processes, plasma enhanced dry chemical etching utilizes ions to attack undesired material on a semiconductor wafer and thereby etch it from the wafer.
One major disabling drawback with most plasma etch or plasma deposition processes is the deposition of undesired material onto the internal quartz reactor walls. In PECVD, the material generated for deposition deposits not only on the semiconductor wafer positioned therein for processing, but everywhere within the reactor. Likewise with plasma etching, the material etched from the wafer and the products of the gas discharge will coat the internal reactor walls as opposed to being entirely pumped from the reactor. These wall-adhering materials can be very chemically stable, and thus are not easily removed by simple dry chemical etching techniques.
It would be desirable to overcome these drawbacks in the development of improved plasma reactors, and methods for cleaning plasma reactors.